Showing papers on "Silica gel published in 2006"

Hydrothermal growth of mesoporous SBA-15 silica in the presence of PVP-stabilized Pt nanoparticles: synthesis, characterization, and catalytic properties.

Abstract: A novel high surface area heterogeneous catalyst based on solution phase colloidal nanoparticle chemistry has been developed. Monodisperse platinum nanoparticles of 1.7−7.1 nm have been synthesized by alcohol reduction methods and incorporated into mesoporous SBA-15 silica during hydrothermal synthesis. Characterization of the Pt/SBA-15 catalysts suggests that Pt particles are located within the surfactant micelles during silica formation leading to their dispersion throughout the silica structure. After removal of the templating polymer from the nanoparticle surface, Pt particle sizes were determined from monolayer gas adsorption measurements. Infrared studies of CO adsorption revealed that CO exclusively adsorbs to atop sites and red-shifts as the particle size decreases suggesting surface roughness increases with decreasing particle size. Ethylene hydrogenation rates were invariant with particle size and consistent with a clean Pt surface. Ethane hydrogenolysis displayed significant structure sensitivi...

Complete CO oxidation over Cu2O nanoparticles supported on silica gel.

Abstract: We find that nearly monodisperse copper oxide nanoparticles prepared via the thermal decomposition of a Cu(I) precursor exhibit exceptional activity toward CO oxidation in CO/O2/N2 mixtures. Greater than 99.5% conversion of CO to CO2 could be achieved at temperatures less than 250 degrees C for over 12 h. In addition, the phase diagram and pathway for CO oxidation on Cu2O (100) is computed by ab initio methods and found to be in qualitative agreement with the experimental findings.

Toward monodispersed silver nanoparticles with unusual thermal stability.

Abstract: A novel in situ autoreduction route has been developed, by which monodispersed silver nanoparticles with tunable sizes could be easily fabricated on silica-based materials, especially inside the channels of mesoporous silica (MPS). 13C CP/MAS NMR spectroscopy was employed to monitor the whole assembly process. It was demonstrated that the amino groups of APTS (aminopropyltriethoxyl silane)-modified MPS can be used to anchor formaldehyde to form novel reducing species (NHCH2OH), on which Ag(NH3)2NO3 could be in situ reduced. Monodispersed silver nanoparticles were thus obtained. In situ XRD and in situ TEM experiments were used to investigate and compare the thermal stabilities of silver nanoparticles on the external surface of silica gels (unconfined) and those located inside the channels of SBA-15 (confined). It was observed that unconfined silver nanoparticles tended to agglomerate at low temperatures (i.e., lower than 773 K). The aggregation of silver nanoparticles became more serious at 773 K. However...

4 – Adsorption and Ion Exchange

Abstract: Publisher Summary This chapter deals with the phenomena of adsorption and ion exchange, and highlights their role in environmental protection. Activated carbon, silica gel, and alumina are the most popular adsorbent materials in industry due to the fact that they provide large surface areas per unit weight. Activated carbon is produced from coconut shell, wood, and bone, whereas silica gel is made of hydrated silicon dioxide. Alumina can be either mined or produced by precipitated aluminum oxide and hydroxide. There are a number of cases where carbon adsorption is preferably used in water treatment—if compounds are not compliant for biodegradation, if the molecule contains branched chairs, has a large molecular weight, and low polarity, and generally, in the case of removing organic and inorganic species, for concentrations lower than 5000 and 1000 mg/L, respectively. Besides the applications of water treatment, activated carbon is called into action for the removal of volatile organic compounds, such as solvents, toxic gases, and odors from gaseous waste streams. Ion exchange shares many characteristics with adsorption, such as mass transfer from the fluid to the solid phase; there are, however, some significant differences. In ion exchange, the ions removed from the liquid phase are replaced by ions from the solid phase. So, there actually occurs an exchange of ions and not only a removal in the latter process. This principle is the basis of use of ion exchange in removing the effects of pollution in air and water.

Acidic ionic liquid modified silica gel as novel solid catalysts for esterification and nitration reactions

Abstract: Covalent bond immobilized ionic liquids are prepared via radical chain transfer reaction of 1-allylimidazolium based ionic liquids on modified silica gel and proved to be recyclable heterogeneous acidic catalysts for esterification and nitration reactions.

Supported ionic liquids. New recyclable materials for the L-proline-catalyzed aldol reaction

Abstract: New materials for L-proline recycling have been developed. These materials have been applied to the L-proline-catalyzed aldol reaction between acetone and several aldehydes. The L-proline has been supported on the surface of modified silica gels with a monolayer of covalently attached ionic liquid with or without additional adsorbed ionic liquid. Good yields and ee values, comparable with those obtained under homogeneous conditions, have been obtained especially with imidazolinium-modified and 4-methylpyridinium-modified silica gels. Moreover, these materials have been easily recovered by simple filtration and studies about their reuse have been carried out. These studies showed that these materials can be used at least up to seven times. Finally, in the case of imidazolinium-modified silica gel, investigations showed that this support can be regenerated giving again a recyclable material with high isolated yields and reproducible ee values up to 13 cycles.

3‐Zinciobenzofuran and 3‐Zincioindole: Versatile Tools for the Construction of Conjugated Structures Containing Multiple Benzoheterole Units

Abstract: General. All the reactions dealing with air or moisture sensitive compounds were carried out in a dry reaction vessel under a positive pressure of argon or nitrogen. Airand moisture-sensitive liquids and solutions were transferred via syringe or stainless steel cannula. Analytical thin-layer chromatography was performed using glass plates pre-coated with 0.25-mm 230‐400 mesh silica gel impregnated with a fluorescent indicator (254 nm). Thin layer chromatography plates were visualized by exposure to ultraviolet light (UV) and/or by immersion in an acidic staining solution of p-anisaldehyde followed by heating on a hot plate. Organic solutions were concentrated by rotary evaporation at ca. 15 Torr (evacuated with a diaphragm pump). Flash column chromatography was performed as described by Still et al., 1 employing Kanto Silica gel 60 (spherical, neutral, 140‐325 mesh). Materials. Commercial reagents were purchased from Tokyo Kasei Co., Aldrich Inc., and other commercial suppliers and were used either distilled or recrystallized before use. Zinc chloride (anhydrous, beads, -10 mesh, 99.99 %) was purchased from Aldrich and was flame-dried just before use. Anhydrous tetrahydrofuran (THF) and diethylether were purchased from Kanto Chemical Co. and distilled from sodium / benzophenone ketyl at 760 Torr under an argon atmosphere immediately before use. TMEDA and toluene were distilled from calcium hydride at 760 Torr under an argon atmosphere immediately before use. Triethylamine was distilled from calcium hydride at 760 Torr under an argon atmosphere and was subsequently stored over 4-A molecular sieves in a storage flask. The water content of the solvent was confirmed with a Karl-Fischer moisture titrator to be less than 20 ppm. The following starting materials were prepared as described in the literature: 2-phenylethynylphenol, 2 2-(1-hexynyl)phenol, 2

ZrOCl2·8H2O/silica gel as a new efficient and a highly water–tolerant catalyst system for facile condensation of indoles with carbonyl compounds under solvent-free conditions

Abstract: ZrOCl 2 ·8H 2 O/silica gel as a highly water–tolerant catalyst system has been applied for the preparation of bis(indolyl)methanes via electrophilic substitution reactions of indoles with carbonyl compounds under solvent-free conditions. The yields of the isolated products are from good to excellent.

Microchip-Based Macroporous Silica Sol−Gel Monolith for Efficient Isolation of DNA from Clinical Samples

Abstract: Effective microchip extraction of deoxyribonucleic acid (DNA) from crude biological matrixes has been demonstrated using silica beads or hybrid phases composed of beads and sol−gel. However, the use of monolithic sol−gels alone for extraction of human genomic DNA has been more difficult to define. Here we describe, for the first time, the successful use of monolithic tetramethyl orthosilicate-based sol−gels for effective micro-solid-phase extraction (μSPE) of DNA in a glass microchip format. A functional monolithic silica phase with micrometer-scale pores in the silica matrix resulted from addition of poly(ethylene glycol), a poragen, to the precursor mixture. This allowed a monolithic sol−gel bed to be established in a microchip channel that provided large surface area for DNA extraction with little flow-induced back pressure. DNA extraction efficiencies for simple systems (λ-phage DNA) were ∼85%, while efficiencies for the reproducible extraction of human genomic DNA from complex biological matrixes (hu...

Applications of silica supports in affinity chromatography

Abstract: The combined use of silica-based chromatographic supports with immobilized affinity ligands can be used in many preparative and analytical applications. One example is the use of silica-based affinity columns in HPLC, giving rise to a method known as high-performance affinity chromatography (HPAC). This review discusses the role that silica has played in the development of affinity chromatography and HPAC and the applications of silica in these methods. This includes a discussion of the types of ligands that have been employed with silica and the methods by which these ligands have been immobilized. Various formats have also been presented for the use of silica in affinity chromatographic methods, including assays involving direct or indirect analyte detection, on-line or off-line affinity extraction, and chiral separations. The use of silica-based affinity columns in studies of biological systems based on zonal elution and frontal analysis methods will also be considered.

KAl(SO4)2·12H2O supported on silica gel as a novel heterogeneous system catalyzed biginelli reaction: One-pot synthesis of di-hydropyrimidinones under solvent-free conditions

Abstract: 3,4-Dihydropyrimidin-2(1H)-ones derivatives were synthesized in moderate to high yields in one-pot three component reaction from the corresponding aldehydes, ketones or 1,3-dicarbonyl compounds and urea, in the presence of catalytic amount of KAl(SO4)2·12H2O (Alum) supported on silica gel (Alum-SiO2) as a non-toxic, reusable, inexpensive and easily available reagent, under solvent-free conditions at 80 °C. Compared to the classical Biginelli reaction, this new method consistently has the advantage of good yields.

Spherical ordered mesoporous silicas and silica monoliths as stationary phases for liquid chromatography.

Abstract: Ordered mesoporous silicas such as micelle-templated silicas (MTS) feature unique textural properties in addition to their high surface area (approximately 1000 m2/g): narrow mesopore size distributions and controlled pore connectivity. These characteristics are highly relevant to chromatographic applications for resistance to mass transfer, which has never been studied in chromatography because of the absence of model materials such as MTS. Their synthesis is based on unique self-assembly processes between surfactants and silica. In order to take advantage of the perfectly adjustable texture of MTS in chromatographic applications, their particle morphology has to be tailored at the micrometer scale. We developed a synthesis strategy to control the particle morphology of MTS using the concept of pseudomorphic transformation. Pseudomorphism was recognized in the mineral world to gain a mineral that presents a morphology not related to its crystallographic symmetry group. Pseudomorphic transformations have been applied to amorphous spherical silica particles usually used in chromatography as stationary phases to produce MTS with the same morphology, using alkaline solution to dissolve progressively and locally silica and reprecipitate it around surfactant micelles into ordered MTS structures. Spherical beads of MTS with hexagonal and cubic symmetries have been synthesized and successfully used in HPLC in fast separation processes. MTS with a highly connected structure (cubic symmetry), uniform pores with a diameter larger than 6 nm in the form of particles of 5 microm could compete with monolithic silica columns. Monolithic columns are receiving strong interest and represent a milestone in the area of fast separation. Their synthesis is a sol-gel process based on phase separation between silica and water, which is assisted by the presence of polymers. The control of the synthesis of monolithic silica has been systematically explored. Because of unresolved yet cladding problems to evaluate the resulting macromonoliths in HPLC, micromonoliths were synthesized into fused-silica capillaries and evaluated by nano-LC and CEC. Only CEC allows to gain high column efficiencies in fast separation processes. Capillary silica monolithic columns represent attractive alternatives for miniaturization processes (lab-on-a chip) using CEC.

Development of cost-effective noncarbon sorbents for Hg(0) removal from coal-fired power plants.

Abstract: Noncarbonaceous materials or mineral oxides (silica gel, alumina, molecular sieves, zeolites, and montmorillonite) were modified with various functional groups such as amine, amide, thiol, urea, and active additives such as elemental sulfur, sodium sulfide, and sodium polysulfide to examine their potential as sorbents for the removal of elemental mercury (Hg(0)) vapor at coal-fired utility power plants. A number of sorbent candidates such as amine- silica gel, urea- silica gel, thiol- silica gel, amide-silica gel, sulfur-alumina, sulfur-molecular sieve, sulfur-montmorillonite, sodium sulfide-montmorillonite, and sodium polysulfide-montmorillonite, were synthesized and tested in a lab-scale fixed-bed system under an argon flow for screening purposes at 70 degrees C and/or 140 degrees C. Several functionalized silica materials reported in previous studies to effectively control heavy metals in the aqueous phase showed insignificant adsorption capacities for Hg(0) control in the gas phase, suggesting that mercury removal mechanisms in both phases are different. Among elemental sulfur-, sodium sulfide-, and sodium polysulfide-impregnated inorganic samples, sodium polysulfide-impregnated montmorillonite K 10 showed a moderate adsorption capacity at 70 degrees C, which can be used for sorbent injection prior to the wet FGD system.

Conversion of rice hull ash into soluble sodium silicate

Abstract: Sodium silicate is used as raw material for several purposes: silica gel production, preparation of catalysts, inks, load for medicines, concrete hardening accelerator, component of detergents and soaps, refractory constituent and deflocculant in clay slurries. In this work sodium silicate was produced by reacting rice hull ash (RHA) and aqueous sodium hydroxide, in open and closed reaction systems. The studied process variables were time, temperature of reaction and composition of the reaction mixture (expressed in terms of molar ratios NaOH/SiO2 and H2O/SiO2). About 90% silica conversion contained in the RHA into sodium silicate was achieved in closed system at 200 °C. The results showed that sodium silicate production from RHA can generate aggregate value to this residue.

Immobilized Polysaccharide-Based Chiral Stationary Phases for HPLC

Abstract: Polysaccharide derivatives, such as phenylcarbamates and benzoates of cellulose and amylose, are known to show high chiral recognition abilities for many racemates when used as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). This type of CSPs has usually been prepared by coating the polysaccharide derivatives onto a macroporous silica gel without a chemical bond. Therefore, rather limited numbers of solvents can be used as eluents, and solvents, such as chloroform and tetrahydrofuran (THF), which swell or dissolve the polysaccharide derivatives cannot be used. The selection of a suitable eluent is very important for both analytical and preparative separations. In order to enhance the versatility of the polysaccharide derivative-based CSPs, the derivatives have to be immobilized. Here we review the immobilization methods of the polysaccharide derivatives mainly onto silica gel.